An orthogonal frequency division multiplexing (OFDM) scheme divides one high-speed data stream into a plurality of low-speed data streams and simultaneously transmits a plurality of low-speed data streams by using subcarriers that are orthogonal to each other, thereby making it possible to increase transmission efficiency. A technology of the apparatus for orthogonal frequency division multiplexing is developed to a current scheme using 2-D signal constellation and 1-D inverse fast Fourier transform in the early 1980's and has been prevalently used for a wireless communication apparatus or a mobile communication apparatus. The signal constellation is a set of signal points and is for transforming the binary bit sequence generated from the information source into signals which have different amplitudes and phases from each other prior to being modulated into radio frequency.
FIG. 1 is a diagram showing 2-D signal constellation used in existing digital communication or a wireless communication system. In more detail, FIG. 1A shows an example of constant amplitude constellation where all the signal points configuring the signal constellation have the same amplitude and FIG. 1B is a diagram showing non-constant amplitude characteristics as 16-ary quadrature amplitude modulation (QAM) constellation where 16 signal points have different amplitudes and phases. FIG. 2 is a diagram for explaining an apparatus for orthogonal frequency division multiplexing to use the existing 2-D signal constellation for signal mapping.
Referring to FIG. 2, the apparatus for orthogonal frequency division multiplexing using the existing 2-D signal constellation includes a serial-to-parallel converter 110, a 2-D signal mapper 120 that is configured by 2-D signal constellation, an inverse discrete Fourier transform unit (IDFT) 130, and a parallel-to-serial converter 140, as shown in FIG. 2.
The serial-to-parallel converter 110 parallelizes serial binary bit sequence input. The 2-D signal mapper 120 maps the parallelized input signals to the signal points of 2-D signal constellation, respectively. The input signals are changed into N complex values corresponding to the signal points of the 2-D signal constellation and are then modulated in the inverse Fourier converter 130, as in the following Equation 1. Next, the modulated N complex values are serialized by the parallel-to-serial converter 140 and are then transmitted.
                              x          n                =                              1            N                    ⁢                                    ∑                              k                =                0                                            N                -                1                                      ⁢                                                  ⁢                                          X                k                            ⁢                              exp                ⁡                                  (                                      ⅈ2π                    ⁢                                                                                  ⁢                                          nk                      /                      N                                                        )                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, 0≦n≦N−1. Where N represents the number of sub-channels used in the apparatus for orthogonal frequency division multiplexing and k represents each sub-channel.
Meanwhile, in the case of the apparatus for orthogonal frequency division multiplexing using the existing 2-D signal constellation, it has been used for the plurality of wireless communication apparatus or the mobile communication apparatus without being almost changed until now after being devised in the approximately same type as the current apparatus in the early 1980's. Therefore, a need exists for a new apparatus for orthogonal frequency division multiplexing capable of showing more excellent performance in view of a symbol error rate and remarkably reducing transmission power required to achieve the same reference symbol error rate, as compared to an apparatus for orthogonal frequency division multiplexing using 2-D signal constellation.